Microelectromechanical systems or MEMS have electromechanical structures typically sized on a millimeter scale or smaller. These structures are used in a wide variety of applications including for example, sensing, electrical and optical switching, and micron scale (or smaller) machinery, such as robotics and motors. MEMS devices are very sensitive to environmental exposure. As such, MEMS structures are encased in hermetically sealed packages.
In the case of optomechanical switches and other opto-coupling devices, optical fibers must enter the package without allowing the interior of the package to be exposed to the environment. This includes possible exposure through any space that may exist between the optical fiber and the sleeving that surrounds the fiber. To prevent this, the sleeving typically is removed and the fiber sealed with a sealing material at an orifice to the package.
The hermetic seal is of particular concern in optomechanical devices. This is because any surface contaminants on the devices can affect mechanic properties, including increasing stiction; can affect optical properties, including reducing the reflectance of optical structures; and can affect electromagnetic interactions. These concerns are in addition to the typical concerns over exposure of electrical components to the environment. Thus, in optomechanical devices, a robust hermetic sealing is of particular importance.
The packages typically are formed of a nickel iron compound plated with a gold exterior layer. A nickel layer can be plated on the nickel iron compound to facilitate adhesion of the gold to the package.
Typically, a solder process is used to hermetically seal the package. One example of such a process is disclosed in U.S. Pat. No. 5,692,086, by Beranek et al., entitled OPTICAL FIBER LOCKING SUBMOUNT AND HERMETIC FEEDTHROUGH ASSEMBLY, issued Nov. 25, 1997, herein incorporated by reference in its entirety. Difficulties with conventional solder processes include fiber degradation caused by higher temperatures of such processes.
Therefore, what is needed is a low temperature solder process which provides a robust hermetic seal and favorable device performance and yields.
In a possible implementation in accordance with the present invention, a device is provided capable of receiving an optical fiber through an orifice in the housing of the device. The housing has a gold surface over a substrate material. An indium layer is located on the gold surface. A solder joint is formed with the indium layer covering the gold surface to have indium silver solder surrounding the fiber and maintaining the fiber in a desired position with the housing.
In one embodiment, the indium layer is deposited by electroplating and is formed of pure indium. The solder joint may be formed of indium silver solder having about 97% indium and about 3% silver.
The indium silver solder can have a melting point below that of the indium layer. This allows the indium layer to act as protective barrier over the gold surface, inhibiting the indium silver solder from dissolving the gold surface. The indium layer adheres well to the gold and the indium silver solder, in turn, adheres well to the indium layer.
Further, the indium silver solder adheres well to the gold surface if some or all of the indium is dissolved during the solder process. If part or all of the indium layer is dissolved, the indium silver solder is less reactive when it does contact the gold surface. As a result, indium silver solder reduces the reaction rate with the gold surface if the indium layer is sacrificed or removed by the indium silver solder. The silver in the indium silver solder functions to reduce the reaction of the indium in the solder with the gold surface. Thus, the indium silver solder further inhibits removal of the gold surface even if the solder comes in contact with the gold surface. This is particularly beneficial in situations where the temperature, or the temperature uniformity or gradient, at the solder site is difficult to control precisely.
As a result, compared with a pure indium solder process not employing a gold protection layer such as an indium layer, the combination of the indium layer with the indium silver solder can increase the process window, from about 1-2 minutes to about 10 minutes. In addition, and as a result of a larger process window, the indium silver solder used in conjunction with an indium layer over the gold surface allows for greatly improved production yields with less process constraints.
In a preferred embodiment, the solder joint formed on the indium layer covering the gold surface provides an improved hermetic seal at both the fiber entrances, and exits, of the device.
It is possible in some embodiments to remove an intermediate portion of the fiber sleeve and place the sleeveless portion within the entrance or exit tunnel through the side wall of a device housing where the solder joint will be formed. Removing an intermediate portion of the sleeve conveniently allows the multiple fibers of a ribbon-type cable to be generally maintained in relative alignment during the solder process. It also conveniently allows alignment during an indium fiber/fibers metalization process if applicable.
The fiber or fibers may be metallized with a pure indium coating. The indium metalization provides good wetting with the indium silver solder and adheres well to a glass fiber. If metallized with pure indium, no additional flux is necessary to provide a good solder joint. Similarly, no flux is necessary on a pure indium layer to provide a good solder joint with the interior tunnel surface.
If the metalization layer is formed by stripping and metallizing a bare fiber, preferably the metalization layer does not extend under the sleeving. If such is the case, a sheath, such as epoxy, may be formed to extend over the sleeving, the bare fiber, and onto an adjacent portion of the metallized fiber for strain relief if desired.
As such, in some embodiments, the sheath seals the interface between the sleeving and the metalization layer, and covers the bare portion of the fiber. It also provides rigidity at the interface between the sleeving and the metalization layer to help protect against breakage of the fiber at this interface during cable installation through the housing wall, and during positioning and soldering of the cable.